I believe +s is setuid. +t is sticky. Sticky used to mean that an
executable would be held in VM rather than being swapped in from disk
all the time (useful for busy executables), however in more modern
usage, +t is used on directories so that only the owner of a file can
unlink it from a sticky directory, despite others having write
permission to the directory.

Whether or not a sticky directory has this interpretation is largely
filesystem-type dependent. You can expect most unix-oriented
filesystems to use sticky this way, but filesystems that originated on
other operating systems are likely to not support it.